Work physiology is the science that studies how the human body responds to the physical stress of work or activity demands. These physiological responses are important in maintaining homeostasis in the body during work activities and reducing the adverse effects of physiological fatigue due to work. Homeostasis is defined as the maintenance of a constant or changing environment. In practical terms, it refers to the relatively constant internal environment of the human body during both stressed and relaxed conditions, due to many regulating anatomical and physiological systems. These organ systems and physiological responses regulate cellular metabolism, energy production, cellular waste product removal, voluntary muscle control, and the flow of blood and oxygen to working muscles. An understanding of the role of major organ systems in the human body during work activities and the relationships between work intensity and recovery intervals is essential to the science of ergonomics.

Metabolism
To accomplish work, the body requires energy, oxygen and nutrients. The human body consumes and uses carbohydrate, fat and protein nutrients to provide the required energy to maintain homeostasis both at rest and during work activity. During work, the primary nutrients utilized are fats and carbohydrates, with proteins contributing less than 5-15% of the total energy used. These nutrients, after having been converted to chemicals, enter the blood stream and circulate to the various internal organs and muscles. At the muscle sites, this chemical energy is converted into mechanical energy, or a muscle contraction, and heat. This process is known as metabolism.

Working muscle requires a constant supply of energy. The fundamental source of energy for these contractions is the high-energy Adenosine Triphosphate (ATP) molecule. The ATP molecule is the most important energy carrying molecule in the muscle cell. The ATP compound consists of three parts: adenosine molecule, a ribose molecule and three phosphate molecules linked together by chemical bonds. The bonds linking the phosphate molecules are high-energy bonds and when these bonds are broken, large amounts of energy are released. This energy is then used for muscle contractions. The energy can be liberated from the ATP molecule by a process known as phosphorylation. This metabolic process is shown below. Phosphorylation is the process in which the Adenosine Triphosphate molecule is broken down by the enzyme ATPase into Adenosine Diphosphate (ADP), a phosphate molecule (Pi) and energy.
Aerobic Metabolism
The Adenosine Triphosphate needed for muscle work can be produced from either aerobic (with oxygen) metabolism or from anaerobic (without oxygen) metabolism. The aerobic metabolism of nutrients refers to the oxidation of glucose or glycogen molecules and fatty acids to form ATP, this process is called aerobic glycolysis. This metabolic pathway requires a continuous supply of blood in order to provide ongoing oxygen and nutrients.

A cardiovascular response to increased workload is to increase the amount of blood flowing to active muscle. However, it can take almost one minute for this response to be activated. Therefore, at the onset of most industrial tasks, or in cases of quick-high intensity tasks, it is not always possible to have adequate blood flow available to working muscles. When this occurs, the muscles switch to anaerobic metabolism.

Anaerobic Metabolism
The muscle cells can produce Adenosine Triphosphate (ATP) or energy, without oxygen (anaerobic metabolism) by two methods: the first method is to break high-energy phosphate bonds in Creatine Phosphate (CP) molecules. The second method is by a process known as anaerobic glycolysis. Under anaerobic conditions, the simplest and thus immediate source of energy is through the use or production of the Adenosine Triphosphate (ATP) molecule by breaking high-energy phosphate bonds in the Creatine Phosphate (CP) molecule. The CP molecule donates a phosphate(P) to an ADP molecule to create an ATP molecule and energy. Creatine Kinase is the enzyme that initiates this reaction in the muscle

The second anaerobic metabolic process for energy synthesis is called anaerobic glycolysis. This process also generates a limited amount of energy, but does so by breaking the chemical bonds in the breakdown of glucose to lactic acid. Anaerobic glycolysis can only produce enough ATP or usable energy for a few minutes. In this method, however, the supply of CP is quickly depleted in under 1 minute. Anaerobic glycolysis provides energy for up to four minute. Only the aerobic glycolysis process can provide a sustained supply of energy to working muscles. With both anaerobic processes, work can only be sustained for short periods because is a limited supply of available ATP and CP molecules in the muscle cells

Muscle Fatigue
When skeletal muscle is continually stimulated, the force or tension that is developed by the muscle fibers diminishes. This failure of muscle fiber to maintain tension as a result of contractile activity is known as muscle fatigue. The onset of fatigue depends on both the type of skeletal muscle fibers as well as the intensity and duration of the muscle contractions. The red muscle fibers, or the -slow twitch- fibers appear to have better blood flow and therefore oxygen supply to maintain aerobic metabolism. In the slow twitch muscle fibers, fatigue develops more slowly. These muscles fibers are used mostly during long duration, low intensity activities. The white muscle fibers, also called -fast twitch- fibers, appear to rely more upon anaerobic metabolism. These fibers fatigue more rapidly, and are used more for short duration, high intensity activities. The development of muscle fatigue corresponds to four events that occur in working muscles:

1.)The depletion of the concentration of ATP. The rate of ATP utilization exceeds the rate of production. The muscle cannot contract without ATP.
2.)Increased amounts of intracellular acidity due to the rise in lactic acid levels. This increased hydrogen ion concentration affects the contractile proteins of the muscle fibers, decreasing the force generated by the muscle fibers.
3.)The depletion of muscle glycogen levels. As the amount of available glycogen diminishes, the muscle can no longer sustain a contraction.
4.)Levels of other metabolic waste products, including Carbon Dioxide, increase within muscle cells. If levels of acid and carbon dioxide waste products build up, this will slow aerobic metabolism, resulting in less efficient metabolism.

If muscle fatigue sets in and the muscle is no longer able to sustain work efficiently, the muscle becomes overloaded resulting in micro trauma to the muscle fibers. If this fatigue and overloading is repetitive or long term in nature the resulting microtrauma becomes cumulative and pathology or injury occurs. Local muscle fatigue is suspect to contribute to work-related Cumulative Trauma Disorders. In order to avoid the adverse effects of muscle fatigue, a sufficient supply or flow of blood to the working muscles is critical.

Since aerobic metabolism generates almost 20 times as much ATP for energy as does anaerobic energy, the effects of muscle fatigue can be minimized by ensuring work load intensity is low enough so that adequate oxygenation, or blood flow to the active working muscles is achieved. If heavy workloads are required, they should be brief in duration, lasting less than a few seconds or minutes, which reduces the effects of prolonged anaerobic metabolism, and maximizes metabolic efficiency.

Summary
The most important factor in ergonomic job design or modification is to promote aerobic metabolism and adequate blood flow, resulting in a high metabolic efficiency. This will maintain adequate blood flow to working muscles, prevent fatigue and allow maximal performance. Dynamic muscle contractions are always preferred over static muscle loading situations. Work-rest cycles should provide sufficient recovery times to sufficiently perfuse active muscles with blood. Jobs should be designed or modified to minimize or reduce the requirements for static contractions, such as static grips, extended reaches and extreme postures.